Wow, you must be awfully patient to wait 4 years to bump your thread!
The guy I bought my 5 window from put in some shoulder belts from a '91 GMC. He attached them by putting in a 3/8 piece of steel behind the interior panel that goes between the rear window and the corner window. I honestly don't know how they'd hold up in a real wreck, but I'm sure they'd be better than nothing.
BTW, what is the grill guard on the Chevy in the website you posted? It would look great on my GMC.